The present invention relates generally to memory circuits, and more particularly, to specialpurpose memory circuits used in digital signal processing systems. There is a general class of computational problems in which numerical data must be sorted in accordance with its relationship to one or more sets of dimensional limits. It is with this class of problems that the present invention is concerned.
For example, in the processing of census data it may be useful to determine the number of people within specified age limits living within a specified geographical area, as defined by a numerical range of postal ZIP codes. Clearly, such a problem is easily handled by a programmable general-purpose digital computer. The speed of operation of the computer is usually of no concern, since there is typically no need for an immediate solution to the problem. However, there are other problems in the same general class that must be solved more rapidly. In some cases, the solution is required in "real time," or almost instantly as a data stream is received and processed. A programmed digital computer may still be fast enough to satisfy the requirements of some of these applications. For others, however, even the fastest programmable computers cannot provide real-time solutions to the complex signal processing problems that are involved.
One area in which real-time solutions are typically required is the field of pattern recognition and image processing. For example, the processing of photographic data from an earth-orbiting satellite may require that areas having a particular color and a particular shape be identified and counted. Pattern recognition has many well known applications in both military and commercial areas.
Another important example of the type of signal processing with which the invention is concerned is the processing of radar pulses. The analysis of radar pulses from multiple sources is frequently too complex for human operators to be able to handle rapidly and reliably. In a typical system, each received radar signal is preprocessed into digital fields that characterize and completely define the signal. The fields might contain such information as time of arrival, frequency, pulse amplitude, pulse width, angle of arrival, and so forth. In a typical application, there may be many such signals whose sources are known and are of no consequence to the purpose of the signal processing. Since the characteristics of these signals from known sources are also known, a first processing step is usually to filter out these known signals from the ones of more particular interest. This filtering step is analogous to finding the number of people in a selected age group in a particular region of the country, but in the radar example the specified items are discarded rather than further analyzed. Both applications, however, are examples of a processing step that might be described environmental filtering.
In any event, it will be appreciated from the foregoing that there is a significant need for an alternative to using programmed computers for the type of environmental filtering steps that are called for in a large number of signal processing applications. The present invention satisfies this need, as will now be further explained.